Mobile communication devices such as Personal Digital Assistants (PDAs) and cellular telephones are typically powered by rechargeable internal batteries. Each different mobile communication device is typically provided along with its own external battery adapter/charger to charge the device's internal batteries. Even for the same type of mobile communication device, a different brand or version of the same type of device often required a different type of adapter/charger. Most of the time, a new version of a device comes with a new version of an external battery adapter/charger because the battery adapters/chargers of the older versions of the device are not compatible with the new version of the device. Therefore, when a new version of a mobile communication device becomes available, previous versions of the device are usually discarded together with their external battery adapters/chargers. This is expensive and harmful to the environment.
The Universal Serial Bus (USB) is a bus that is now supported on many mobile communication devices. Often, the batteries of a mobile communication device can be recharged through the USB port of the device without having to use an external adapter/charger. For example, the batteries of a cellular telephone can be recharged by connecting to the USB port of a desktop computer that is turned on. The batteries of the cellular telephone can also be recharged by connecting to the USB port of a laptop computer that is turned on. The laptop computer is powered by an AC adapter that is in turn plugged into an AC wall socket. Alternatively, if there is no AC wall socket available, then the laptop computer is powered by its internal batteries.
When an external AC power supply is not available, and when the batteries of the mobile communication device are discharged to a low level, it is often desirable to be able to charge the cellular telephone through its USB port from the battery inside a laptop computer. However, it usually takes more than one hour to fully charge the cellular telephone. A powered-on laptop computer consumes a substantial amount of power even if its CPU is sitting in the idle state. Therefore, due to the amount of time it takes to recharge the batteries of the cellular telephone, the CPU of the laptop consumes a large portion of energy available from its battery during the recharge period. This is especially a problem in situations when the cellular telephone needs to be charged right away but the battery of the laptop computer is inadequately charged or the available charge needs to be saved for later use.